Measuring current is always a nuisance because you have to break the circuit to put the measuring device in series with the circuit. That problem never goes away. Still, any high-end digital multimeter can accurately measure currents down into the microampere and high-nanoampere range. But the process gets tricky as the current levels fall into the low-nanoampere and picoampere ranges.
At these levels, noise becomes a real problem. Also, the internal impedances of the measuring instrument or circuit become extremely critical to ensure an accurate measurement. By following a few key guidelines, though, designers can achieve accurate low-current measurements with moderately priced test equipment.The Shunt Ammeter Approach
Typical digital multimeters (DMMs) make low-current measurements by passing the current to be measured (IIN) through a precision resistor (RS) known as a shunt and then measuring the voltage across it (Fig. 1). The voltage produced across it by the current to be measured is called the burden voltage (Vburden). The burden voltage is an error source since it subtracts from the applied voltage. This causes the current flow to be less than the actual value, thereby producing an error.